Research is directed toward the elucidation of the mechanism of genetic recombination. We are attempting to understand the role played by recombination genes and enzymes in other functions, particularly cell growth and viability and the repair of radiation-induced DNA damage. These problems are being approached by continuing our studies of a series of isogenic strains of Escherichia coli we have constructed, which contain mutations in the rec, sbc, uvr, and polA genes, singly and in various combinations. Specific experimental questions being pursued include the following: 1. What specific biochemical defects are associated with the low viability of cultures of recA minus and recB minus strains? Studies of unfractionated cultures as well as fractions enriched for the dead cells are under way. How do the known enzyme defects in these mutants relate to low viabilities observed? 2. What is the map location of the sbcA minus class of Rec plus revertants? What is the product of this gene and how is it involved in recombination? Detailed studies of these Rec plus revertants in comparison to the rec plus wild type are under way. 3. What is the mechanism of repair of UV induced DNA damage carried out by the recombinational system in the rec plus and sbcA minus revertants? 4. How are the uvr (excision) and rec mediated UV repair systems coordinated? 5. We are continuing purification and study of properties of the recA protein and its involvement in genetic recombination. 6.We will purify the ATP independent DNase found in sbcA minus (Rec plus) revertants and compare its properties to the recB recC wild type DNase product.